This invention relates to a control apparatus for an internal combustion engine, and in particular to a control apparatus for an internal combustion engine which is operable either with a low-octane rating fuel or a high-octane rating fuel.
It is well known in the art that the octane rating of fuel gasoline is greatly interrelated with a knock-resistibility in an internal combustion engine. Namely, as the octane rating of gasoline becomes higher, the knocking of an engine correspondingly decreases.
FIG. 1 shows an ignition timing vs output shaft torque characteristic in an internal combustion engine in the case where a commercially available low-octane gasoline (regular gasoline) and a commercially available high octane gasoline (premium gasoline) of which the octane rating is higher than that of the low-octane gasoline. In the figure, point A indicates a knock limit for the low-octane gasoline while point B indicates a knock limit for the high-octane gasoline, in which knock arises when the angle of the ignition timing is advanced beyond these points. According to FIG. 1, it is possible to advance the angle of the ignition timing up to a point B if the high-octane gasoline is used, where the output shaft torque will be higher as compared with the use of the low-octane gasoline.
FIG. 2 shows a supercharging pressure vs output shaft torque characteristic of an engine associated with a supercharger. Similar to FIG. 1, point A represents a knock limit when using the regular low-octane gasoline and point B represents a knock limit when using the premium high-octane gasoline. It will be seen from FIG. 2 that the increase of a supercharging pressure when the premium gasoline is used for the engine enables the output shaft torque of the engine to be also increased as in FIG. 1. Therefore, in an engine associated with such a characteristic supercharger, if the ignition timing and the supercharging pressure are advanced and increased respectively according to the mixing ratio of the regular gasoline and the premium gasoline in the case where their mixture is used or either one is used, the output of the engine can be enhanced.
However, in a conventinal ignition timing control apparatus for an internal combustion engine, the reference ignition timing characteristic is preset only for a predetermined gasoline, e.g. the low-octane rating gasoline. Therefore when using the high-octane rating gasoline, an increase in the output of the engine cannot be expected without any change or modification of the reference ignition timing characteristic or the reference supercharging characteristic, so that by some method, the presetting of the reference ignition timing must be repeated towards the advance angle or the presetting of the supercharging pressure must be repeated towards the high pressure.
Particularly, such a repeated presetting of the reference ignition timing is very hard when using the mixed fuel because, as shown in FIGS. 1 and 2 by point C, the knock limit exists between points A and B according to the mixture ratio and so the possible advance limit or the possible increased pressure limit is variable.
Even if the reference ignition timing or the supercharging pressure has been again preset at the knock limits A and B, since the knock limits A and B in FIG. 1 may vary with environmental conditions during the operation of the engine such as temperature, humidity, etc., knocks easily arise at the time of a transient operation such as an accelerated operation of the engine, and accordingly it is quite difficult to avoid the occurrence of engine knocks.
U.S. Pat. No. 3,822,583, titled "Method for Determining Octane Ratings of Fuels under Road Conditions" issued on July 9, 1974 to B. D. Keller et al. discloses a basic ignition timing control according to the octane rating of gasoline by a knock detection.